If you want to grind, Exponential Growth is good enough to get you to the point where you don't need to grind anymore, since by then you'll be stronger than most of the enemies.

Exponential Growth does break the difficulty of Lunatic in half, though, and you might also just want to do the character DLC when you're strong enough, since that doesn't have such a high reward:effort ratio.

It's meant to discourage grinding, just like how Spotpass EXP got cut. I think they probably would have done it for DLC too, but decided not to, making stuff you pay for not work would create a lot of rage.

Lunatic is a lot of fun, though. You see yourself slowly get better at a chapter and feel really accomplished once you do beat it.